Photo-induced magnetization in copper(II) octacyanomolybdate(IV)

Abstract

The photomagnetic effect of the mixed-valence compound Cu 2 II[Mo IV(CN) 8]·7.6H 2O (Cu II; 3d 9, S=1/2, Mo IV; 4d 2, S=0) ( 1) was investigated. The UV–VIS spectra showed the intervalence transfer (IT) band between Mo IV–CN–Cu II and Mo V–CN–Cu I around 500 nm. When the paramagnetic compound 1 was irradiated by a filtered blue light (400–430 nm, 3 mW/cm 2) of a Xe lamp at 5 K, the irradiated sample exhibited a spontaneous magnetization with a Curie temperature of 17 K. By annealing the irradiated sample above 200 K, the magnetic property of this sample returned to the initial state. This photo-induced magnetization is explained by the following mechanism. The excitation of the IT band causes the electron transfer from Mo IV ( S=0) to Cu II ( S=1/2), producing a mixed-valence isomer of Mo V ( S=1/2)–CN–Cu I ( S=0). However, half of the copper ions remain as Cu II ions due to the stoichiometric limitation and hence the irradiated 1 is expressed as Cu ICu II[Mo V(CN) 8]·7.6H 2O. The unpaired electrons on Mo V ( S=1/2) ions and those on Cu II ( S=1/2) ions in the irradiated compound interact ferromagnetically, giving rise to the spontaneous magnetization.